public class TestMenuExample extends MenuHolder {

  private final MenuButton exampleButton;

  public TestMenuExample(Player player) {
        super(main or simpleplugin.getinstance, 54);

        exampleButton = new MenuButton() {
            @Override
            public void onClickInsideMenu(Player player, Inventory menu, ClickType click, ItemStack clickedItem, Object object) {
                    //do your own code
            }

            @Override
            public ItemStack getItem() {
               //use this as you want.
                return new ItemStack(Material.HOPPER);
            }
        };

  public ItemStack getItemAt(int slot) {
    if (slot == 5)
    return exampleButton.getItem();
    return null;
  }
 }
}